LIDAR (Light Detection and Ranging) is technology for observation of atmospheric states through the irradiation of an atmospheric region for observation with short-pulse laser light, and the measurement and analysis of the scattered light as a LIDAR signal. An Nd:YAG laser is generally used to emit the laser light for LIDAR observation. Types of scattering resulting from laser light irradiation include Mie scattering due to aerosols (suspended particles), Rayleigh scattering by component molecules of the atmosphere, and Raman scattering; these can be employed in analyses of the atmospheric temperature, spatial distribution of aerosols, atmospheric density, concentration distribution of atmospheric components, and other observed elements. In practice, such analyses are performed by solving equations, called LIDAR equations, which represent the LIDAR signals.
Mie scattering is a scattering phenomenon which is caused by substances of sizes approximately equal to the wavelength of the irradiated laser light. Rayleigh scattering is a scattering phenomenon caused by substances smaller than the wavelength of the irradiated laser light. Mie scattering signal components and Rayleigh scattering signal components are superposed in LIDAR signals from observation regions in which aerosols exist; in order to perform high-precision measurements using Rayleigh scattering LIDAR, Mie scattering signal components must be efficiently separated or blocked (spectrally analyzed). Techniques for spectral analysis of Mie scattering signal components generally employ iodine absorption filters as filters with high blocking rates. Spectral analysis techniques employing iodine absorption filters involve sweeping the single-frequency light wavelength so that the wavelength of the irradiated laser light matches a specific absorption spectra line of the iodine gas, then irradiating the atmosphere with the laser light, and guiding the LIDAR signal to the iodine absorption filter. By this means, the Mie scattering signal component contained in the LIDAR signal is absorbed by the iodine absorption filter, and only the Rayleigh scattering signal component is passed.
In addition, when a substance is irradiated with laser light, scattered light is observed at a wavelength different from that of the incident light. This is called Raman scattering. Raman scattering LIDAR is effective for observation of atmospheric temperatures and observation of water vapor distributions. For example, when analyzing atmospheric temperature, vibrational Raman scattering of nitrogen molecules and similar is used to determine atmospheric density, and analyses similar to that for Rayleigh scattering LIDAR is performed, or atmospheric temperature may be determined from the intensity distribution of rotational Raman scattering spectra. When analyzing water vapor distribution, vibrational Raman scattering by the water molecules constituting the water vapor and by the nitrogen molecules in the atmosphere is analyzed simultaneously, the rate of attenuation due to the atmospheric density is corrected, and the water vapor distribution is derived from the distribution of the average density of nitrogen molecules.
In order to perform high-precision observations using Rayleigh scattering LIDAR, the superposed Mie scattering signal component and Rayleigh scatting signal component must be analyzed efficiently. In the stratosphere at a height of 30 km or higher, the entire signal is due to Rayleigh scattering by molecules comprised by the atmosphere, and measurement is performed relatively simply; but in the troposphere, there exists intense Mie-scattered light due to aerosols, and the scattering intensity is not uniform. Thus, in order to perform LIDAR observation using Rayleigh scattering, for practical purposes, a Mie scattering signal component blocking rate extending to several decimal places is required.
On the other hand, when using spectral analysis techniques employing an iodine absorption filter, there may be the problem that the transmission characteristics of such absorption filters are unstable. This may arise mainly because the density distribution of the iodine gas in the absorption filter is not uniform, so that there may be instability in the absorption spectra characteristics.
In order to increase the finesse, which is a measure of the spectral resolution, of such a high blocking-rate filter, the filter is coated with a dielectric multilayer film with high reflectivity. However, if the reflectivity is increased through such a coating, the maximum transmittance of the filter is lowered, resulting in a critical drawback for the purpose of LIDAR observations entailing measurements of weak LIDAR signals.
In a conventional LIDAR system for meteorological observation, laser light sources suited to each individual observed element are employed in order to analyze the states of various observed elements. That is, in a conventional LIDAR system for meteorological observation, it has not been possible to use a single laser light source to simultaneously perform various LIDAR observations, to analyze the states of various observed elements. For example, in order to determine the water vapor distribution, use of differential absorption methods or vibrational Random scattering is effective. However, in the former differential absorption method, laser light at two particular wavelengths is necessary, and it was not possible to use these two different types of laser light for observation of other elements. Further, in the subsequent vibrational Raman scattering, the intensity of vibrational Raman scattering by water vapor is weaker by approximately three orders of magnitude than the intensity of Rayleigh scattering, and so in order to improve a signal-to-noise (S/N) ratio of the observation system, a high-output laser and large aperture optical telescope became necessary. As a result, the equipment size was increased, and cost performance was degraded. Also, in spectral analysis methods using the above-described using the above-described iodine absorption filter, there is an absorption spectra line in iodine near the second harmonic component (λ=532 nm) of Nd:YAG lasers, and so there may be the further problem that the laser light wavelength is constrained by absorption line.
Further, because laser light sources are used in LIDAR observations it is necessary to take sufficient account of safety for human beings (eye-safe characteristics). As such, in LIDAR observations, while selecting laser light which enables efficient observation of various scattering phenomena as described above, it is also necessary to develop a system which takes eye-safe characteristics into consideration.
Therefore, an object of the present invention is to propose a new LIDAR system for meteorological observation and a meteorological observation method using same, in order to resolve the problems described above.
More specifically, a first object of the present invention is to realize a LIDAR system for meteorological observation which is capable of efficiently and stably separating or blocking superposed Mie scattering signal components and Rayleigh scattering signal components, in order to perform high-precision observations using Rayleigh scattering LIDAR.
Further, a second object of the present invention is to realize a LIDAR system for meteorological observation which is capable of using a single laser light source to simultaneously perform various LIDAR observations and analyze the states of various observed elements.
Furthermore, a third object of the present invention is to realize a LIDAR system for meteorological observation which, while capable of efficient LIDAR observations, also takes eye-safe characteristics into consideration.